Emitted when the notification is closed.
Note that when a [class@Notify.Notification] is used in a sandboxed environment where XDG Desktop Notification Portal is implicitly used,
the signal [signal@Notify.Notification:NotifyNotification:closed] is only emitted when the notification is closed in
response to an user action response.
NO signal will be emitted if the user or the daemon dismissed the notification for any other reason.